In order to reproduce acoustic ambience over a wide range of frequencies with adequate fidelity and power, the choice of loudspeakers must take into account the conditions relating to their attachment and their installation.
A loudspeaker generally includes a motor, or drive mechanism, and a membrane or cone driven by the drive assembly so that it vibrates in order to reproduce sound over a certain band in the audio-frequency range, between 30 and 16000 Hertz. The assembly is held together and protected by a frame structure or basket assembly.
The membrane for the loudspeaker is generally circular. It is attached by its edge to the basket, and is driven at its centre by the coil of the drive assembly, causing it to vibrate, preferably at a frequency (F) corresponding to a vibration that has an antinode at the centre and a node at its perimeter, the distance between the antinode and the node being at least a quarter of the wavelength (λ) of the vibration.
It can be shown that the diameter (D) of the membrane corresponds to the vibration frequency (F) of the wavelength (λ) such that:D=c/2*F=λ/2where c is the speed of sound.
To increase the bandwidth of the membrane around frequency F, a non-resonant material, such as paper, is employed for the membrane, and in addition the latter is molded so that it has ridges at its periphery in order to isolate it as well as possible from the flare of the basket and to smooth out the position of the nodes.
To cover a wide range of frequencies, it is also necessary to resort to loudspeakers of different diameters (D1, D2, etc.) corresponding to centre frequencies F1, F2, etc. This leads to the design of loudspeakers known as tweeters (D<8 cm), mid-range (8 cm<D<16 cm), bass or woofers (16 cm<D<25 cm), and subwoofers (D>25 cm). Finally, to reduce the volume of these loudspeakers, they have to be mounted coaxially.
Moreover, and independently of the installation conditions, it is necessary to ensure that the whole range of frequencies thus covered by means of a multiplicity of loudspeakers is distributed throughout the listening volume, such as a living room or an office used as an auditorium, possibly in stereo thus doubling the number of loudspeakers or enclosures necessary. It is this distribution problem that is surmounted by the invention, as will be shown below.
Oddly enough, it was not this problem that the inventor was trying to solve, but rather a different problem, which arose in the very different context of creating a vibrating physical-therapy appliance for use in the medical area, with the opposite function to a stethoscope, and employed to treat internal organs that had developed mechanical malfunctions (incontinence, constipation, etc.).
To solve the medical problem, the inventor had the idea of drawing upon expertise developed at least twenty-five years ago but abandoned at the time, relating to the problem of vibration-diffusion. The solution concerned what one could describe as a loudspeaker with no membrane or cone.
This loudspeaker had an electromagnetic drive assembly with a cylindrical magnetic core and a voice coil surrounding the core and attached to a set of two plates—a flexible, centering suspension plate ensuring the radial positioning of the coil around the core while also allowing the axial vibration of the coil, and a rigid plate designed to be fixed, for example, to a wall of an auditorium. When this latter plate was attached, the wall replaced the membrane traditionally used in a loudspeaker. However, the output of the drive assembly and the acoustic quality of the loudspeaker were much too low. For a drive unit of 35 watts, the result was a sound of mediocre quality and very low level—some 60 dBA.